Human respiratory syncytial virus (RSV) is a pneumovirus of the paramyxovirus family and the most common cause of bronchiolitis and pneumonia in infants under one year of age. Most children become infected with RSV prior to their second birthday resulting in 75-125,000 hospitalizations. The associated medical costs are thought to exceed $650 million annually in the United States alone. In addition, early-life, respiratory viral infections, notably with RSV, increase the risk of the subsequent development of childhood asthma (Holt and Sly, 2002). RSV infection can produce severe, lower respiratory tract disease in patients of any age. The elderly, as well as those having compromised cardiac, pulmonary or immune systems, are particularly vulnerable and it is estimated that some 14,000 deaths occur annually in the United States in subjects over 65 years old. In addition, RSV infection is increasingly regarded as an important precipitator of exacerbations in patients suffering from chronic obstructive pulmonary disease (COPD) (Mohan et al., 2010) as well as asthma (Newcomb and Peebles, 2009) and cystic fibrosis (Abman et al., 1988). In immuno-compromised adults, approximately 50% of upper respiratory tract infections with RSV progress to pneumonia.
The initial portal of entry by RSV is through the nose or eye rather than the mouth (Hall et al., 1981). Once established in the upper respiratory tract the virus is able to migrate readily into the lungs. The pathophysiology of RSV infection was investigated in a study of lung tissues obtained from deceased children (Johnson et al., 2007). Examination of tissues from four individuals revealed immunostaining of epithelial cells indicating the presence of RSV, without basal cells being affected. The epithelial localization of the pathogenic organism provides a challenge to treatment since a supra-effective concentration of the drug substance has to be maintained at the discrete cellular site to enable the infection to be treated and subsequently cleared.
The RSV virus exists as two antigenic sub-groups: A and B. Viruses of the RSV A strain were formerly regarded as the sub-group pathogens responsible for the majority of clinical disease and were reported to produce a more symptomatic pathology (Walsh et al., 1997; Panayiotou et al., 2014). A common RSV A strain is RSV A2 (Olivier et al., 2009). However, during a recent outbreak in China virus strains from the RSV B sub-group were found to predominate in the afflicted population (Zhang et al., 2010).
Over the last two decades considerable progress has been made in the treatment of a number of viral diseases including human immunodeficiency virus (HIV) and both hepatitis B and hepatitis C. In all these cases gold standard therapies have evolved that consist of combination treatments that were brought about, at least to some extent, in response to the emergence of drug resistant disease.
FDA-approved drugs for the treatment of acute RSV infections comprise of (aerosolised) ribavirin and the humanized, monoclonal antibody, palivizumab (Synagis). The latter agent targets the RSV fusion (F) protein and is limited to prophylactic use in high risk, paediatric patients. Furthermore, clinical variants resistant to neutralization by palivizumab were recently identified (Zhu et al., 2011) and therefore no truly effective vaccine is currently available. The use of ribavirin is limited by its low potency against the virus and by concerns over its side-effect profile. Consequently there is an urgent, unmet need for the discovery of novel, safe and effective therapies against RSV infection having an improved clinical profile. Moreover, in view of the emerging prominence of the RSV B strains in clinical disease it is highly desirable that these treatments be efficacious against infections arising from both RSV A and RSV B strains.